General Double-Zero-Index Photonic Crystals.

Some photonic crystals (PCs) with Dirac-like (or semi-Dirac) conical dispersions exhibit the property of double-zero index (DZI), which was previously thought to occur only at the center of the Brillouin zone (Γ point). Here, we demonstrate a general DZI effect in PCs, whose Dirac-like point can appear at multiple high-symmetry positions, with the previous conventional DZI at the Γ point being a special case. We theoretically elucidate the physical mechanism of the general DZI and summarize its necessary and sufficient conditions. We then experimentally confirm the photonic band and DZI effect in a general DZI PC with a semi-Dirac cone at the X point and demonstrate its unique transmission characteristics, including a 180° phase shift upon transmission, wave front flattening, and waveguiding with natural zero radiation loss. The extraordinary properties of general DZI PCs may open new avenues for designing directional light sources, phase-controlled optical switches, nanolasers, and on-chip photonic circuits.